In a non-destructive inspection method using a magnetic field, a metal material, which is an inspection target object (inspection target), is irradiated with the magnetic field by energization to an excitation coil or a permanent magnet, a difference in magnetic field distribution caused by a characteristic change (hereinafter, referred to as a defect) such as a crack or a material change is detected by a magnetic sensor, and the defect is detected. For example, a leakage magnetic flux method, an eddy current flaw detection method, and the like are known. The leakage magnetic flux method is to infiltrate a magnetic flux into an inside of an inspection target using a direct current (DC) magnetic field or low frequency excitation in which magnetic field intensity does not generally change with time and to detect the magnetic flux leaking from a metal, which is the inspection target, in a vicinity of a crack by the magnetic sensor. The eddy current flaw detection method is to generate a magnetic flux of which a current value changes with time by supplying a current of which a current value changes with time to an excitation coil of an eddy current probe, to generate an eddy current by bringing the eddy current probe close to the metal, which is the inspection target, and to obtain a change of the eddy current as a detection signal by the magnetic sensor.
As a method of detecting a change of the magnetic field, in addition to a method of mechanically scanning a surface of the inspection target using a magnetic sensor such as a coil or a hall sensor, there is an inspection method which introduces a detection method for mapping spatial distribution of the magnetic field as a two-dimensional image using a magneto optical effect. For example, PTL 1 (JP-A-2014-153318) discloses a technique in which a magnetic thin film is disposed as a magnetic transfer film on an inspection surface of the inspection target, response light, which is reflected light obtained by irradiating the magnetic thin film with light, is imaged by a camera, and magnetic field distribution changed by the defect is acquired as an image and evaluated.